A weighing device of the generic type addressed by the invention, for example a device for weighing objects of a uniform kind, finds application primarily in automated production—and test systems, where scales of a modular design, so-called weighing modules are particularly well suited for incorporation in such systems. Frequently, the weighing modules employed here, which are also often referred to as weighing cells, are of a type where the indicator unit is arranged in separation from the weighing module or weighing cell, as is the case for example in a system with a central indicator for a plurality of such weighing modules or weighing cells. Areas of application for integrated weighing modules of this kind are installations for the production and/or testing of small, relatively expensive parts, for example in filling—and packaging machines for tablets, capsules ampoules, etc., in the pharmaceutical industry, or for the checking of ball bearings. The weighing of uniform objects and also the so-called batch weighing are processes where multiple loads have to be weighed individually in a confined space for a diversity of purposes such as checking, dispensing measured doses, and/or fill-weighing.
Systems of this kind which serve to weigh uniform objects belong to the known state of the art. The prevalent types are linear arrays of weighing cells. Other arrangements are based on a concept of spreading the weighing cells out in a two-dimensional layout like satellites associated with a linear array of load receivers, wherein the latter have to match the spacing of the conveyor elements of an existing conveyor system, as the individual weighing modules or weighing cells are in many cases too large to be lined up at the required small intervals.
In a weighing cell which operates according to the principle of electromagnetic force compensation, the force generated by a load on the weighing pan is compensated by a force-compensating member consisting of a permanent magnet and a coil, wherein the current is measured which flows through the coil and generates the compensating force. This measurement value is in proportion to the weighing load. However, this measurement value is also dependent on the position of the coil in the magnetic field of the permanent magnet, which is why the coil always has to be in the same position relative to the magnet at the moment when the measurement value is captured. The position of the coil after applying a load is determined by a position sensor, and the current through the coil is increased until the displacement of the coil which was caused by the load is compensated. Now the coil current is measured which is representative of the weight of the applied load. A weighing cell of this kind is disclosed in U.S. Pat. No. 4,280,577, wherein the weighing cell includes between the load receiver and the force-compensating member a force-transmitting mechanism whereby the force generated by the load on the load receiver is transmitted to the force-compensating member, either reduced or amplified in magnitude depending on the load range.
A weighing cell operating according to the same principle is disclosed in U.S. Pat. No. 4,099,587. In this patent, the load receiver is coupled directly to the force-compensating member through a force-transmitting rod. Attached to the force-transmitting rod is the movable portion of the position sensor, while the stationary part of the position sensor is rigidly connected to the housing-mounted part of the weighing cell, specifically to the part of the force-compensating member that has a fixed position relative to the housing. This arrangement, which is referred to as the direct-measurement principle, is used with preference in the range of small weighing loads.
A weighing cell that is designed for installation in a receiving holder arrangement is described in WO 2007/041979 A1. In order to establish a releasable mechanical and electrical connection with an associated receiving holder arrangement of an appropriate complementary design, the weighing cell has alignment elements which are designed to cooperate with alignment elements of the receiving holder arrangement. The receiving holder arrangement is intended for permanent installation in a machine that is designed to perform weighing functions.
In cases where it is important to quickly measure the individual masses of weighing objects, it suggests itself for example to use a setup of several weighing cells to allow the individual masses such as weighing objects of a uniform kind to be weighed in parallel. A need exists in particular for weighing systems that are suitable for use in the area of mass determination in the range of milligrams to grams. In the case of weighing cells whose dimensions are large in comparison to the spacing between the conveyor elements of a conveyor system, and with the use of a complex conveyor system, in particular of a kind where the conveyor elements are set up in a two-dimensional arrangement, even a star-shaped arrangement as disclosed in the prior art will not meet the requirements. As the dimensions of the weighing cells, length as well as width, are often larger than the required spacing between the central longitudinal axes of their load receivers, only a limited number of weighing cells can be placed around an area provided for the arrangement of the load receivers which is determined by the conveyor elements of the conveyor system.